The Math Class and Wrapper Classes

Math class methods (static)

Method	Returns
Math.abs(x)	absolute value (int or double)
Math.pow(a, b)	a to the power b, as a double
Math.sqrt(x)	square root, as a double
Math.random()	a double in [0.0, 1.0)

Random integers

To get a random int from min to max inclusive:

int r = (int) (Math.random() * (max - min + 1)) + min;

A die roll: (int) (Math.random() * 6) + 1 gives 1 to 6.

Math.random() * 10 gives values from 0.0 up to but not including 10.0, so (int) of it is 0 to 9.

Wrapper classes

Integer and Double wrap primitive values so they can be stored in collections such as ArrayList.

• Autoboxing: Integer n = 5; converts int to Integer automatically.
• Unboxing: int m = n; converts back.
• Integer.MAX_VALUE and Integer.MIN_VALUE give the int limits.
• Integer.parseInt("42") converts a String to an int; Double.parseDouble("3.5") to a double.

Common pitfalls

• Math.pow and Math.sqrt return double; assigning to an int needs a cast.
• Comparing two Integer objects with == compares references; use equals or unbox.

Quick summary

• Math methods are static and called with the class name.
• Random int in [min, max]: (int)(Math.random() * (max − min + 1)) + min.
• Autoboxing and unboxing move between int and Integer.